Gas flow past the mouthpiece during the expiration phase in an infant ventilator of the type described above is important to both sweep exhaled gases from the mouthpiece to ensure that the infant will not inhale his own exhaled gases, since the mouthpiece holds a large volume of gas compared to the volume of an infant's breath; and to provide gas which the infant may voluntarily inhale during the expiratory phase, since it is difficult to design a ventilator that can be switched to an inspiration phase by an infant's weak attempts to inhale.
Typically in the type of infant ventilator described above, the gas delivery system includes a flexible hose through which the gas flows past the mouthpiece and to an outlet end of the hose spaced from the mouthpiece. An outlet valve is positioned at the outlet end of the hose, which outlet valve is movable via control means for the ventilator between (1) a closed position blocking the outlet end of the hose so that gas pressure at the mouthpiece will increase sufficiently to cause involuntary inspiration of an infant to which the mouthpiece is coupled (the inspiration phase of the ventilator), and (2) an open position so that gas will flow past the mouthpiece and through the outlet end of the hose to the atmosphere (the expiration phase of the ventilator).
Problems arise in such prior art infant ventilators when the hose between the mouthpiece and the outlet is occluded, as can happen when the hose is inadvertently crimped. Known prior art infant ventilators include no means for shutting off the inflow of gas through the gas delivery system when such an occlusion occurs so that the gas pressure at the mouthpiece can quickly reach dangerous levels. Those known prior art ventilators deal with this hazard by providing a spring-loaded relief valve positioned in the gas delivery system upstream of the mouthpiece, which relief valve will vent incoming gas flow to the atmosphere when pressure in the delivery system exceeds a predetermined set level (generally the maximum pressure the gas deliver system will normally apply to an infant). Should such an occlusion persist with those prior art infant ventilators, an infant being ventilated will be subjected to sustained high pressure, which situation is potentially very hazardous.